1. Field of the Invention
This invention relates to a toner for developing an electrostatic image, and an image forming method, used in recording processes that utilize electrophotography, electrostatic recording, magnetic recording or the like. More particularly, this invention relates to a toner for developing an electrostatic image, and an image forming method, used in copying machines, printers, facsimile machines and so forth in which a toner image is previously formed on an electrostatic latent image bearing member and the toner image is thereafter transferred to a transfer medium to form an image.
2. Related Background Art
A number of methods are conventionally known for electrophotography. Final images such as copies or prints are commonly obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a toner to make it visible to form a toner image, transferring the toner image to a transfer medium such as paper if necessary, and thereafter fixing the toner image to the transfer medium by heat, pressure or heat-and-pressure.
In recent years, there is an increasing demand for color image formation in image forming apparatus such as copying machines, printers and facsimile machines employing electrophotography. As color toners, non-magnetic toners are commonly used since it is difficult to use magnetic toners containing magnetic materials, in relation to their tints. In instances where a magnetic toner is used as a black toner and a non-magnetic toner is used as a color toner, optimum transfer current value of the non-magnetic toner tends to be higher than optimum transfer current value of the magnetic toner. If transfer conditions of the machinery main body are adjusted to the non-magnetic toner, the magnetic toner may cause a phenomenon called "re-transfer", in which the toner once transferred to a transfer medium returns onto the latent image bearing member, to cause a decrease in image density of black images.
In recent years, paper materials are being made available in greater variety, and hence the copying machines, printers and facsimile machines employing electrophotography are sought to be adaptable to such various paper materials. However, optimum transfer conditions may differ depending on paper materials serving as transfer medium. For example, cardboards and overhead projector films (OHP films) have a high optimum transfer current value and thin paper has a low optimum transfer current value. Hence, if the transfer conditions of the machinery main body are made optimum to cardboards or OHP film, the phenomenon of "re-transfer" may also occur when transferred to thin paper.
Japanese Patent Application Laid-open No. 61-279864 discloses a toner whose shape factors SF-1 and SF-2 are defined. However, this publication has no disclosure at all as to transfer. Also, as a result of experiments to follow up Examples, using toners described therein to carry out transfer, the toner has been found to have an insufficient transfer efficiency, and must be more improved.
Japanese Patent Application Laid-open No. 63-235953 also discloses a magnetic toner whose particles have been made spherical by a mechanical impact force. However, the toner still has an insufficient transfer efficiency, and must be more improved.
As printers, LED printers and LBP printers are prevailing in the recent market. As a trend of techniques, there is a tendency toward higher resolution. That is, those which hitherto have a resolution of 240 or 300 dpi are being replaced by those having a resolution of 400, 600 or 800 dpi. Accordingly, with such a trend, the developing systems are now required to achieve a higher minuteness.
Copying machines have also made progress to have higher functions, and hence they trend toward digital systems. The digital systems chiefly employ a method in which electrostatic latent images are formed by using a laser, and hence, the copying machines also trend toward a high resolution and, like the printers, it has been sought to provide a developing system with high resolution and high minuteness.
From such viewpoints, especially in the printers and copying machines of digital systems, their photosensitive layers are increasingly made thinner so that electrostatic latent images can be formed in a higher minuteness. When such thin-film photosensitive members are used, the electrostatic latent images have a low potential contrast, and hence toners used in development are desired to be toners having a higher developing performance.
In recent years, from the viewpoint of environmental protection, there is a tendency that, in place of the primary charging process and transfer process utilizing corona discharge as conventionally used, a primary charging process and a transfer process which employ a photosensitive member contact member is prevailing.
For example, proposals are disclosed in Japanese Patent Application Laid-open No. 63-149669 and No. 2-123385. These are concerned with a contact charging method and a contact transfer method. A conductive elastic charging roller is brought into contact with an electrostatic latent image bearing member, and the electrostatic latent image bearing member is uniformly electrostatically charged while applying a voltage to the conductive elastic roller, followed by exposure and developing steps to obtain a toner image. Thereafter, while another conductive elastic transfer roller to which a voltage is applied is pressed against the electrostatic latent image bearing member, a transfer medium is passed between the electrostatic latent image bearing member and the conductive elastic transfer roller to transfer to the transfer medium the toner image held on the electrostatic latent image bearing member, following by the step of fixing to obtain a transferred image.
However, in such a contact transfer system that utilizes no corona discharge, the transfer member such as a roller is brought into contact with the electrostatic latent image bearing member via the transfer medium at the time of transfer, and hence the toner image is pressed when the toner image formed on the electrostatic latent image bearing member is transferred to the transfer medium, so that a problem of partly faulty transfer tends to occur, which is called "blank areas caused by poor transfer".
In addition, as toners are made to have a smaller particle diameter, the attraction (image force, van der Waals force or the like) of toner particles to the electrostatic latent image bearing member becomes larger than the Coulomb force applied to the toner particles during transfer, so that the toner remaining untransferred tends to increase.
Moreover, in the roller transfer charging system, the physical and chemical action on the surface of the electrostatic latent image bearing member, attributable to the discharge caused between the charging roller and the electrostatic latent image bearing member, is larger than that in the corona charging system. Hence, especially when a combination of organic photosensitive member/blade cleaning is employed, the roller tends to wear because of a surface deterioration of the organic photosensitive member to cause a problem on its lifetime. However, when a combination of contact charging/organic photosensitive member/one-component magnetic development/contact transfer/blade cleaning is employed, the image forming apparatus can be made low-cost, and small-size and light-weight with ease. Thus, such a system is prevailing in copying machines, printers and facsimile machines in the field where low prices, and small size and light weight are needed.
Accordingly, toners and photosensitive members used in such image forming methods have been sought to have superior release properties.